The invention relates generally to the field of improved microlens molds and microlens. More particularly, the invention concerns a method of making a precision mold suitable for forming high quality, micro-sized optical articles, such as a microlens or microlens array.
Rotationally symmetric optical surfaces in molds for injection molding or compression molding are typically made either by grinding or diamond turning. While these techniques work well for larger surfaces, they are not suited for making high quality optical surfaces in small sizes or arrays. Other techniques are available for making small scale single lenses and arrays but they are limited as to fill factor, optical accuracy and/or the height or sag of the lens geometry that can be made.
Grinding relies on an orbital motion of the grinding surfaces to make a precision optical surface without scratches. However, the orbital motion and the grinding surfaces become impractical when making optical surfaces below a few millimeters in size. Grinding multiple surfaces for an array can only be done one surface at a time with multiple pieces that are then fit together.
Diamond turning can be used to make optical surfaces down to 2 millimeters in size but the setup is difficult. Precise location of multiple optical surfaces is not possible due to multiple setups. The need for multiple setups also increases the machining time for an array so that diamond turning becomes cost prohibitive.
Another technique that is suitable for making microlenses under 2 millimeters is polymer reflow. Polymer reflow is done by depositing drops of polymer onto a surface and then heating the polymer to allow it to melt and reflow into a spherical shape under the influence of surface tension effects. In order to obtain a truly spherical optical surface, reflow lenses must be separated from one another so that they contact the underlying surface in a round pattern. To maintain round pattern of each lens at the surface, the lenses must be separated from one another which substantially limits the fill factor in an array. U.S. Pat. No. 5,536,455, titled, xe2x80x9cMethod Of Manufacturing Lens Array,xe2x80x9d by Aoyama, et al., Jul. 16, 1996, describes a two step approach for making reflow lens array with a high fill factor. Using this technique, a second series of lenses is deposited in the gaps between the first set of lenses. While this technique can provide a near 100% fill factor, the second set of lenses does not have round contact with the underlying surface so that the optical surface formed is not truly spherical. Also, reflow techniques in general are limited to less than 100 microns sag due to gravity effects. Aspheric surfaces cannot be produced using polymer reflow.
Grayscale lithography is also useable for making microlenses under 2 millimeters. Grayscale lithography can be used to make nearly any shape and high fill factors can be produced in lens arrays. However, reactive ion beam etching and other etching techniques that are used in gray scale lithography are limited as to the depth that can be accurately produced with an optical surface, typically the sag is limited to under 30 micron.
High sag lenses are typically associated with high magnification or high power refractive lenses that are used for imaging. High power refractive lenses have tight curvature and steep sides to maximize the included angle and associated light gathering or light spreading which implies a high sag. In the case of image forming, refractive lenses are preferred to preserve the wave front of the image. In other cases such as illumination where the wave front does not have to be preserved, Fresnel or diffractive lenses where the optical curve is cut into segmented rings, can be used to reduce the overall sag of the lens. In the case of microlenses, high power diffractive lenses are not feasible due to the steepness and narrow spacing of the ring segments at the edge that would be required to make a low sag, high power microlens.
U.S. Pat. Nos. 5,519,539, titled, xe2x80x9cMicrolens Array With Microlenses Having Modified Polygon Perimeters,xe2x80x9d by Hoopman et al., May 21, 1996 and 5,300,263, titled, xe2x80x9cMethod Of Making A Microlens Array And Mold,xe2x80x9d by Hoopman et al., Apr. 5, 1994, describe a method for making lens arrays that involves casting a polymer into a series of small receptacles so that surface tension forms the polymer surfaces into nearly spherical shapes. A correction is done on the shape of the receptacles to make the surfaces more closely spherical but this results in football-shaped intersections so that optical quality and the effective fill factor are limited.
Therefore, a need persists in the art for a method of making a precision microlens mold suitable for forming high quality, micro-sized optical articles, such as a microlens or a microlens array.
It is, therefore, an object of the invention to provide a method of making a precision mold for microsized optical articles.
Another object of the invention is to provide a method of making a precision mold that does not damage the mold surface.
Yet another object of the invention is to provide a method of making a mold that utilizes a cutting member that is not limited to depth of penetration.
Still another object of the invention is to provide a method of making a precision mold that is useable for forming an array of micro-sized optical articles.
It is a feature of the invention that a forming element having a high speed, rotatable half-radius diamond cutting member rotatably engages a substrate in a predetermined cutting pattern to form a precision mold surface in the substrate.
According to one aspect of the present invention, a method of making a microlens array mold, comprises the steps of:
(a) providing a movable substrate;
(b) providing a forming element having a rotatable hardened cutting member for forming said microlens array mold in said movable substrate;
(c) moving said movable substrate to a first milling position relative to said rotatable hardened cutting member;
(d) milling said first milling position to form one of a plurality of microlens molds in said microlens array mold;
(e) removing said rotatable hardened cutting member from said one of a plurality of microlens molds;
(f) moving said movable substrate to expose a second milling position;
(g) moving said rotatable hardened cutting member into a milling relationship with said second milling position;
(h) milling said second milling position of said movable substrate to form another one of said plurality of microlens molds in said microlens array mold; and,
(i) repeating steps (f)-(h) at sequentially different milling positions on said movable substrate to form said microlens array mold having a plurality of microlens molds.
In another aspect of the invention, a microlens and a microlens array made by the method of the invention has a spherical shaped surface, an aspheric shaped surface or an anamorphic shaped surface.
The present invention has the following advantages: the precision microlens mold can be used to mold high quality, micro-sized optical articles, such as microlenses, that have symmetric surfaces with steep sides and high sags; and, the forming element is contoured to produce very accurate optical surfaces in single microlenses or arrays. In the case of arrays, near 100% fill factor can be achieved in the molded article.